Known ringer circuits comprise a loudspeaker receiving signals from an amplifier, the amplifier having a melody input which receives a melody signal, and a power supply input connected to a ringer line which provides a ringer signal that is derived, via chopper power supply means, from the ringing current output by the telephone exchange. The melody signal current varies between maximum and minimum values which are constant and which cause the amplifier to operate in saturation. The operation of such circuits poses many problems. A first problem consists in obtaining sufficient acoustic power, i.e. at least equal to a required minimum, when the ringer signal is at its nominal voltage, while still ensuring satisfactory operation when the voltage of the ringer signal is below its nominal value. It has been observed that, when suitably adjusted for a ringer signal of nominal voltage, for example of the order of 80 volts, existing circuits have a tendency to emit a sound of acoustic power that varies due to lack of electrical power whenever the voltage of the ringing signal from the exchange is low, for example of the order of 25 volts, as happens in particular when the telephone set is connected to a long line with two or three other telephone sets in parallel. This phenomenon of varying acoustic power is referred to as "motorboating". This problem is becoming worse nowadays because the standards for ringer signal voltage vary according to country, so that a ringer circuit which is properly adjusted for the standards of one country may be poorly adjusted for a neighboring country. It is therefore now necessary to provide ringer circuits which differ considerably according to country.
A second problem consists in obtaining linear variation in the emitted acoustic power when the ringer circuit is connected to a unit for setting loudness, such a unit usually acting on the gain of the amplifier. In existing circuits that do not take the power of the ringer signal into account, loudness is allocated with respect to the power available at the nominal voltage of the ringer signal, so that if the ringer signal is not at its nominal voltage, then the available power is completely used up at low loudness settings and a change in setting tending to increase loudness above the level corresponding to the available power has no effect on the acoustic power provided by the loudspeaker, or may even give rise to motorboating, since the power demanded is then much greater than the power available.